It is well known, that because of its particular type of operation, an arc lamp requires very special power supply conditions. Three stages can be distinguishsed in the operation of such a lamp. Firstly, in order to switch the lamp on, it is necessary to apply a high voltage thereto, preferably an alternating voltage, and having an amplitude of about 9 kV to 12 kV.
Once the arc has been struck, it is necessary, temporarily, to deliver high current at low voltage in order to bring the lamp as quickly as possible up to its nominal temperature (and this requirement is particularly important for car lighting).
Finally, once this temperature has been reached, the lamp must be powered at a similar voltage but with lower current.
One known circuit that attempts to come close to the above conditions comprises an alternating voltage generator for generating a few tens of volts at variable frequency. The output from the generator is fed to an LC resonant circuit and the lamp is connected to the terminals of the capacitor.
For striking, the frequency of the generator is chosen to be close to resonance of the LC circuit. An appropriately raised voltage then appears across the terminals of the lamp. Once the arc has been struck, the frequency of the generator is lowered so as to move away from resonance conditions, thereby reducing the voltage across the terminals of the lamp.
Such a circuit suffers from a major drawback, in particular when used in motor vehicles, in that when the voltage is increased for the purpose of striking the arc, a large current flows simultaneously through the inductor and the capacitor.
In practice, the power consumption during this stage rises to about 250 watts, and if the lamp does not light immediately, then the time this stage lasts is incompatible with the amount of electricity available on board a vehicle.
In addition, such a circuit requires a capacitor which is capable of withstanding the high voltage applied to the lamp, and as this is disadvantageous both with respect to cost and with respect to bulk.
The present invention seeks to mitigate these drawbacks of the prior art and to provide a power supply circuit which consumes relatively little power during the arc-striking stage (only a few tens of watts), and which does not require a capacitor having a very high breakdown voltage. More generally, the present invention seeks to provide a circuit which is cheap, which uses components that are simple and not expensive, and which is nevertheless very flexible in providing the voltages required for the various operating stages.